Before getting deeper into repairs...
Have you tried replacing the date/time battery? It's a small, silver battery in a slide out compartment inside the main batteyr compartment of the camera. Those batteries normally have a service life of roughly 5 years, and are relatively cheap, common and widely avalable, and easy to replace. I don't know that a failed date/time battery would cause an Err 99. Usually the only symptom of one is that the camera loses the date/time setting each time you turn it off and back on. Most Canon I've used will shoot fine, even without a date/time battery installed... so this is only a remote possibility, with the problem you're seeing.
Have you tried the camera with a different lens? It could be that the fault lies in the lens... Perhaps there's a short or one of it's components is failing and drawing too much power. There are several electromechanical servos and/or motors in most lenses, driving the aperture, focus and IS, if it's a stabilized lens. There are "flex connectors" in most lenses, too, that can crack with age and use, then short out.
You didn't mention what lens is on the camera. I've seen certain older third party lenses cause errors and lock up cameras, too. For example, I have an older Sigma 28-75mm that works fine on Elan 7 and EOS-3 film cameras and 10D DSLR, but the moment it tries to focus will immediatly cause any later model Canon body to lock up and throw off an error code. It's probably an issue with the firmware of the lens. Old Sigma are the most common at fault, but some old Tamron seem to do it too. Sometimes the manufacturer will update the lens... but sometimes not. This particular 28-75 is a cheap version and not possible to fix.
Have you inspected the memory card socket? Look closely for bent, broken or otherwise damaged pins down in there. Those can cause short circuits in the camera, too.
If none of the above helps, but the camera powers on normally (only goes to Err 99 when the shutter is pressed), you also might try "rebooting" the camera. This is much like rebooting a computer. To do this, first turn the camera off, then remove all the batteries (including the small date/time batt), then turn the camera back on and press the shutter release button once. Nothing will appear to happen. The shutter won't actually fire. This is only being done to drain any remaining power from the circuits.
Next turn the camera off, reinstall the batteries, turn it back on and then navigate the menu to the date/time. If you got a proper reboot, those should need to be reset. (Other menu and custom function settings may have reverted to factory defaults, too. So check those before starting to shoot with the camera again.)
If able to turn the camera on and access it in the menu, you also might want to update (if needed) or reinstall the firmware. A fresh copy of the firmware can be downloaded from the Canon website. It's not difficult, but does have to be done properly, so very carefully follow the instructions you'll find at the Canon website.
Neither a reboot nor an update/reinstall of firmware will solve every problem. But, sometimes they do and they are free to try and easy to do yourself.
If all else fails, you'll have to find someone who works on 30D to find out if it's worth repairing. Not sure if the Canon Service Dept. still supports the 30D... But, if not, there are some good third party repairers out there, too. I've heard a lot of positibe things about http://www.discountcamerarepair.com/home.html on another forum. There are likely others.... some of which may be close to you (Discount is in Denver, CO).
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
